1. Field of the Invention
The present invention relates generally to oil well cementing compositions and methods for their use and more specifically to fluid loss additives for such compositions.
2. Description of the Prior Art
In well completion operations, a casing or liner is suspended in the borehole, and cement is pumped down the casing. The cement flows back up the annulus between the outside of the casing and the wall of the borehole. One problem encountered in this operation is the loss of fluid from the cement slurry into the earth contacting the cement. Such fluid loss causes the dehydrated cement slurry to have reduced strength and an uncontrollable setting rate. In addition, fluid loss can result in damage to the adjacent strata formation.
Polymer compositions have long been added to cementing compositions as a fluid loss additive. For example, U.S. Pat. No. 4,015,991 discloses a fluid loss additive for a hydraulic cement slurry consisting of hydrolyzed copolymers of acrylamide (Am) and 2-acrylamido-2-methyl propanesulfonic acid (AMPS.RTM., a trademark of Lubrizol Corporation). U.S. Pat. No. 4,555,269 discloses the use, as a fluid loss additive, of a copolymer of N.sub.1 N.sub.1 dimethyl-acrylamide and 2-acrylamido, 2-methyl propane sulfonic acid. However, Am/AMPS copolymers are not efficient at reducing fluid loss at bottom hole circulating temperatures (BHCT) ranges of 200.degree.-500.degree. F. Moreover, these copolymers are not particularly suitable for use in saltwater cement slurries.
U.S. Pat. No. 4,674,574 discloses a terpolymer as a fluid loss agent. These terpolymers are comprised of an unsaturated polybasic acid, a nonionic monomer, and a vinyl containing alkyl or aromatic substituted sulfonate.
U.S Pat. No. 4,806,164 discloses a fluid loss additive comprising a polymer of styrene and AMPS.RTM.. This polymer exerts a fluid loss preventative effect at test temperatures up to 180.degree. F.
U.S. Pat. No. 4,700,780 discloses a fluid loss additive that is effective in the presence of substantial salt concentrations. This fluid loss additive comprises a polymer or polymer salt or AMPS.RTM., styrene, and acrylic acid.
The prior art methods are deficient in that none of these methods function well in high temperature wells, i.e. from 200.degree.-500.degree. F. Thus, the need exists for a cement fluid loss additive for a cement fluid loss additive that will function in high temperature wells and preferably is compatible with high salt concentrations.